Congressional forecasters say a Senate bill that aims to repeal and replace Obamacare would leave 22 million more people uninsured by 2026. That's only slightly fewer than a House version that passed last month. This forecast comes as Senate Republican leaders press for a vote on the bill later this week, and it has already led one Republican senator to firmly oppose the bill. NPR's Scott Horsley joins us now. And, Scott, these numbers come from the Congressional Budget Office, the nonpartisan bean counters on Capitol Hill. So where do they think these coverage reductions are coming from?

SCOTT HORSLEY, BYLINE: Ari, the biggest drop would be in Medicaid. Remember, Obamacare expanded Medicaid. This bill would shrink it. And the forecasters anticipate by 2026 you would have 15 million fewer Americans getting their coverage through that safety net program. They're also anticipating a drop of about 7 million people getting coverage through the individual marketplace. And the Congressional Budget Office says these drops would disproportionately affect older people, especially those between 50 and 64 years old, at the lower end of the income ladder.

SHAPIRO: Senate Republican leader Mitch McConnell hopes to get a vote on this bill by the end of the week before lawmakers go home for their Fourth of July recess. How does this forecast affect the chances of the bill passing?

HORSLEY: Well, it doesn't help. Just this evening, since this report came out, we've had one Republican senator - Susan Collins, a moderate Republican from Maine - say definitively that she will not vote for this Senate bill. She says, I want to work with my Republican and Democratic colleagues to fix the flaws in the Affordable Care Act. The CBO analysis shows the Senate bill won't do it. And remember, Ari, that Republicans have very little margin for error here. The Democrats are united in their opposition to this bill, so Republican Senate leader Mitch McConnell can only afford to lose two votes and still pass the measure. With Collins out he's down to one.

SHAPIRO: Senators made some last-minute changes to the bill to address a potential problem that critics said could have led to further erosion in the individual insurance market. Tell us about what they did.

HORSLEY: The original Senate bill, which was unveiled last week, required insurance companies to offer coverage to everyone, including those with pre-existing medical conditions, but it didn't have any kind of requirement for individuals to have health insurance. And critics say that had the potential to really destabilize the market by creating a perverse incentive for people to wait till they got sick in order to go out and buy insurance coverage, which would have the effect of driving up costs for everyone.

So the revision, which was issued today, tries to solve that problem by imposing a penalty on those who don't maintain continuous insurance coverage. Basically, if you let your insurance coverage lapse for a little over two months in any given year, the next year you would be shut out of the insurance market for six months. The CBO estimates that modification in the bill would slightly increase the number of people buying insurance in the individual market compared to what the bill would do without it.

SHAPIRO: And this bill would not only roll back the Affordable Care Act. It would also make major changes to Medicaid, the federal health care program for the poor and disabled. What does this Congressional Budget Office estimate say about that?

HORSLEY: Yeah, for the first time, the Senate bill would cap federal spending on Medicaid in 2020. And that would force state governments to either scale back the program or make up the difference out of their own budgets. And then starting in 2025, the Senate bill assumes that medical bills will grow more slowly than they have historically. And if that assumption turned out to be wrong, the states would face an even bigger budget gap. Now, because this CBO report mostly looks at the next decade - that is, the period between now and 2026 - those savings in the out years for the government don't really show up. But they would be big. And this would presumably, in decades to come, push more people off the Medicaid rolls.